In manufacture of a silicon single crystal, the Czochralski method (the CZ method) using a vitreous silica crucible has been employed. In this method, a seed crystal is contacted to the melt surface while rotating in a horizontal direction on a silicon melt surface at a high temperature of approximately 1420° C. which is the melting point of silicon, then pulled gradually to manufacture a single crystal; and a vitreous silica crucible of high purity is used in order to store the silicon melt.
In recent years, a diameter of the silicon single crystal has been increased due to a demand for efficiency of semiconductor device processes. As a result, the diameter of the vitreous silica crucibles has also been increased. The size of vitreous silica crucibles is, for example, 28 inches (about 71 cm), 32 inches (about 81 cm), 36 inches (about 91 cm), and 40 inches (about 101 cm) in diameter. A crucible with a diameter of 101 cm is a enormous crucible having a weight of about 120 kg, and the mass of silicon melt contained therein is 900 kg or more. That is, during the pulling of silicon single crystal, 900 kg or more silicon melt of about 1500 degrees C. is contained in the crucible. As a result, a distance from an external carbon heater to the center of the silicon single crystal, and an amount of melted polysilicon are increased, which causes the temperature in the vitreous silica crucible to become higher. Moreover, the pulling time may be prolonged, and a pulling may last 2 weeks or more. In order to maintain the solid-liquid interface of the silicon melt central part which is contacted with the single crystal near the silicon melting point of 1420 degrees C., the temperature of the vitreous silica crucible is set as high as 1450-1600 degrees C. In the pulling of silicon single crystal which may last 2 weeks or more, the extent of deformation of sidewall sagging of a rim portion of the vitreous silica crucible may be 5 cm or more.
Brown cristobalite is generated on the inner surface of the vitreous silica crucible when contacting with the silicon melt at a high temperature for a long time. As the pulling of a silicon single crystal proceeds, cristobalite grows in a horizontal direction and vertical direction with respect to the inner surface of the vitreous silica crucible to form a ring-shaped spot (brown ring). The formed brown ring is likely to peel off. The peeled-off brown ring is conveyed into the silicon single crystal when falling/mixing in the silicon melt. As a result, the pulled-up silicon ingot is polycrystallized, and the single-crystal yield is reduced.
Bubbles contained in the inner surface of the vitreous silica crucible are also a main cause of decrease in single-crystal yield. As erosion of the inner surface of the vitreous silica crucible proceeds, the bubbles in the inner surface of the vitreous silica crucible enter the silicon melt. The single-crystal yield is reduced by the bubbles in the silicon melt being included in the silicon ingot. In addition, under a high temperature condition for a long time, bubbles contained in the inner surface of the vitreous silica crucible expand significantly. The expanded bubbles cause a deformation of the vitreous silica crucible and a nonuniform inner surface. As a result, a melt surface vibration occurs in the silicon melt, and the single-crystal yield is reduced.
In order to solve such a problem, for example, Patent Literature 1 proposes a method for pulling of silicon single crystal stably by limiting the number of brown rings in a predetermined position within a certain range. In addition, Patent Literature 2 discloses that an amorphous component ratio of the vitreous silica crucible is identified by using a Raman laser.